1. Field of the Invention
The present invention relates generally to the electrical circuitry, and more particularly to digital to analog current generator (IDAC).
2. Description of the Related Art
Current generators historically have employed two different types of electrical circuit designs. The first is a simple current mirror circuit such as that shown in FIG. 1, where transistor 101 has a reference current constantly flowing therethrough. Resistor 104 is provided to set the reference current, and resistor 105 conducts an output current equal to the reference current provided the two transistors 101 and 102 are matched. Voltage source 103 is sufficient to provide a reference gate to source voltage at the gate of transistor 101 and is sufficient to maintain the reference current through the drain of transistor 101. Transistor 102 mirrors the reference current in the right branch of the circuit as shown so that the drain current in transistor 102 is the same as that of the left branch. With similar or identical transistor sizes and a single voltage 103, the resultant current in both right and left branches is identical. However, as a result of the design shown in FIG. 1, drain to source voltage variations across the transistors 101 and 102 can vary widely and produce uncontrolled and unpredictable variations in the resultant output current flowing in the drain of transistor 102. Certain applications, such as MRAM (Magnetic Random Access Memory), cannot employ a current mirror such as the current mirror shown in FIG. 1 due to the resultant wide variations in output current.
Another solution employed for current generator circuit uses a feedback amplifier. FIG. 2 illustrates a design using a feedback amplifier. From FIG. 2, reference voltage source 201 is provided with resistor 202, feedback amplifier 203, and current source transistor 204. In this design, a reference voltage is placed outside the feedback loop and is selected to establish a desired output current through the load resistor 202. Output current at the source of transistor 204 corresponds to the current passing through resistor 202. The feedback amplifier 203 continually adjusts the gate to source voltage of transistor 204 to minimize the effect of gate to drain voltage variations in transistor 204 and thereby maintain a desired output current in load resistor 202.
Control of the current in the design of FIG. 2 depends directly on the absolute value of resistor 205 and reference voltage source 201. While the value of the reference voltage source 201 may be precisely controlled with a DAC (digital to analog voltage converter) the magnitude of the output resistor 205 may not be known or well controlled and can again produce uncontrolled and unpredictable variations in the resultant output current. Certain applications such as MRAM cannot employ a current generator such as the current generator shown in FIG. 2 due to the resultant wide variations in output current.
It would be advantageous to provide a precision current generator that can be used in advanced applications, such as MRAM applications, where the output current is precisely controlled and thus decreases the risk of producing uncontrolled or unpredictable variations in output current, thereby resulting in lower circuit currents, efficient use of power, and generally improved performance.